1. Field of the Invention
The present invention generally relates to a development device used in an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions, a process cartridge incorporating the development device, and an image forming apparatus incorporating the development device.
2. Description of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include a latent image carrier on which an electrostatic latent image is formed and a development device to develop the latent image with developer.
There are image forming apparatuses that employ a magnetic brush development method using two-component developer consisting essentially of toner and magnetic carrier. Development devices that employ the magnetic brush development method generally include a developer container such as a casing in which two-component developer is contained, an agitator such as a conveyance screw to transport the developer inside the casing, and a developer carrier such as a development roller. In the magnetic brush development method, the toner (i.e., toner particles) is electrostatically adsorbed to the carrier (i.e., carrier particles), and the development roller conveys the toner electrostatically attracted to the carrier to a development area, where the development roller is partly exposed through an opening formed in the casing of the development device and faces the image carrier. In the development area, the toner particles adhere to the latent image formed on the image carrier, thus developing it into a toner image.
For example, the developer carrier includes a cylindrical development sleeve, and a magnetic field generator such as a magnetic roller or multiple magnets are provided inside the development sleeve to generate magnetic fields for causing the developer (i.e., developer particles) to stand on end on and get piled on a surface of the development sleeve. More specifically, the carrier particles stand on end on the surface of the development sleeve along lines of magnetic force generated by the magnet roller, and the electrically charged toner particles adhere to the carrier particles, thus forming a magnetic brush. The magnet roller has multiple magnetic poles generated by multiple magnets shaped like bars, for example. The multiple magnetic poles includes an attraction pole for attracting the developer to the development sleeve, a main development pole for transferring the toner to the image carrier, and a release pole for releasing the developer from the development sleeve. In particular, the main development pole is provided at the same circumferential position as a position of the development sleeve that faces the development area. The developer that stands on end on the surface of the development sleeve can be transported circumferentially by rotating at least one of the development sleeve and the magnet roller.
Typically, to facilitate conveyance of the developer, the surface of the development sleeve is sandblasted or bead-blasted so as to form grooves or irregularities in its surface. In particular, development rollers having a grooved surface or irregular surface are widely used in multicolor image forming apparatuses such as copiers and printers for attaining high quality images. Abrading the surface of the development sleeve by forming grooves or sandblasting can prevent or reduce slippage of the developer on the surface of the development sleeve and accumulation of the developer thereon, thus preventing a decrease in image density resulting from it.
For example, JP-2009-80447-A proposes using a development sleeve having multiple recesses formed at regular intervals in the surface of the development sleeve. The density of the developer carried on this development sleeve is as dense as that carried on the sandblasted development sleeve having the irregular surface. Simultaneously, this development sleeve has an ability to transport the developer as high as that of the grooved development sleeve.
FIG. 30 is an enlarged view of an area of about 2 to 3 cm2 of the surface of the development sleeve disclosed in JP-2009-80447-A, viewed from a side of the development device. As shown in FIG. 30, multiple small oval groves or recesses 139Z are formed in the surface of the development sleeve, and each recess 139Z is oblique to a rotary axial direction of the development sleeve with its left end tilted upward and its right end tilted downward. The small recesses 139Z are formed densely on the surface of the development sleeve with a cutting tool such as a tool bit by properly setting the rotational frequency of the bit, the velocity at which the bit is moved, and the rotational velocity of the development sleeve. When the development sleeve rotates downward in FIG. 30, the developer carried on the development sleeve is moved in the direction indicated by arrow Y1 shown in FIG. 30 in a developer release portion, where the developer is separated from the development sleeve, because the recesses 139Z are oblique to the rotary axial direction of the development sleeve. Thus, a force for transporting the developer to one end (in FIG. 30, to the left) of the development sleeve in the rotary axial direction is generated. When the force exerted by the development roller for transporting the developer is in the direction identical to the direction in which the agitator transports the developer while agitating it, the rotational frequency of the agitator can be reduced, thus reducing the energy. Alternatively, when the rotational frequency of the agitator is maintained, the concentration of the toner in the developer can become more uniform.
In such development devices, if the electrical charge of the toner is insufficient due to deterioration of the developer or because the toner is insufficiently mixed with the carrier, the toner cannot be adsorbed to the carrier sufficiently and might be separated from the carrier in the development area or a developer conveyance path through which the developer is conveyed to the development area and collected from the development area. As a result, the toner might leak out the development device and scatter inside or outside the image forming apparatus. In particular, in image forming apparatuses using removably installable development devices, it is preferred that the amount of scattering toner be minimized on the front side in a direction in which users or service persons insert the development device into the image forming apparatus because they also operate or visually check the apparatus there.
Another matter regarding such development devices is carry-over of developer, which is a phenomenon of the developer that is not separated from the developer carrier in the release portion but is carried further by the developer carrier. If the developer after image development, in which the concentration of toner is decreased, is not returned to the developer container but is carried over and supplied again to the development area, image density can become uneven.